NetworkServices use direct access to wicked when running in installation

inst-sys is not running systemd nor sysV init so "systemctl" cannot be
used. Direct access to network backend is used.

Currently only wicked is assumed to be used as network backend in
inst-sys.
Updated changelog
Refactoring: Turned some NetworkServices globals into constants
Added logging into NetworkServices::run_wicked
Merge pull request #169 from mchf/NI-modifications

Use wicked tool for restart / reload in installation mode